The present invention generally relates to semiconductor devices and more particularly to an optical semiconductor device including semiconductor optical amplifier, laser diode, optical modulator, and the like.
In optical communication networks, there is a demand for the optical semiconductor device used therein such as a semiconductor optical amplifier, laser diode, optical modulator, and the like, in that the optical semiconductor device operates stably and at high speed. In the optical semiconductor device that includes therein an optical amplification part such as semiconductor optical amplifier or laser diode, in particular, reflection caused at the exit end surface serving for optical window tends to invite adversary effect on the stable operation thereof due to amplification of the feedback light caused by the reflection, and thus, it has been practiced in the optical semiconductor devices used in conventional optical telecommunication networks to provide an anti-reflection structure at such an exit end surface. Similar problem arises also in the optical modulator that includes an optical absorption layer.
Patent Reference 1
Japanese Laid-Open Patent Application 2003-69149
Patent Reference 2
Japanese Laid-Open Patent Application 2005-223300
Patent Reference 1 discloses a laser diode of the construction that includes an active region and in which there is further provided an anti-reflection structure at an exit end surface of the mesa stripe forming an optical waveguide structure.
In Patent Reference 1, the regions at both lateral sides of the mesa stripe are buried with a semiconductor stack structure that includes therein a p/n carrier blocking layer, wherein there is formed an optical window region at the forward end of the mesa stripe as a part of the waveguide structure constituted by the mesa stripe, with a shape such that the height of the optical window region increases in the forward direction from the forward end of the mesa stripe. It should be noted that such an optical window region is formed by using a mask pattern of the shape that increases a width in the forward direction from the forward end of the mesa stripe.
With the laser diode of such a structure, the laser beam exited from the mesa stripe at the forward end thereof increases the beam diameter when the laser beam has entered into the foregoing optical window region. As a result of such increase of the beam diameter, it becomes possible to decrease the proportion of the light injected back into the mesa stripe after causing reflection and forming a backward light, even when the laser beam is reflected back at the exit end surface of the optical window region.
On the other hand, with the construction of the Patent Reference 1, there arises a problem, in view of the fact that the regions at the lateral sides of the mesa structure constituting the laser diode are buried with the p/n carrier blocking layers, in that there is caused an increase of parasitic capacitance. It should be noted that such increase of the parasitic capacitance invites difficulty for the laser diode to operate at high speed.
Further, with the construction of Patent Reference 1, the direction of extension of the mesa stripe formed on the (100) surface is limited in the [011] directions. When there is caused a minute deviation from these specific directions, there arises a problem that the buried layers form eaves defined by the (111)A surface on the mesa stripe by growing in the direction to face each other.
FIG. 1 shows an example of such eaves.
Referring to FIG. 1, there is formed a mesa stripe 2 on a substrate 1 of InP, or the like, such that the mesa stripe 2 extends in the <0-11> direction, for example. At both lateral sides of the mesa structure 2, there are formed buried layers 3A and 3B of InP, or the like.
With such a structure, the buried layers 3A and 3B creep up over the mesa stripe 2 from both lateral sides thereof, and thus, there the buried layers 3A and 3B form eaves 3a and 3b. Thereby, it should be noted that each of the eaves 3a and 3b has a surface of (111)A orientation at the lower side thereof facing the top surface of the mesa stripe. There are cases in which such eaves 3a and 3b extend up to the height of 1 μm or more, for example in the waveguide of so-called high-mesa structure, in which the height of the mesa stripe is increased for suppressing reflection of the light guided through the mesa stripe by the top surface of the mesa stripe.
Thus, with the laser diode of Patent Reference 1, the extension direction of the mesa stripe is restricted strictly in the [011] directions, and there is no degree of freedom in the design of the laser diode. Further, with such a laser diode, there is a tendency that the yield of production is degraded.
On the other hand, Patent Reference 2 teaches that such formation of eaves can be suppressed when a material of organic chlorine-containing substance is added at the time of deposition of the buried layer.
Thus, the inventor of the present invention has conducted a series of experiments for suppressing the formation of eaves in the investigation that constitutes the foundation of the present invention, by forming the buried layers by an MOCVD process while adding organic chlorine-containing substance at the time of burying the mesa stripe of the optical semiconductor device with a buried layer. With this investigation, a semi-insulating semiconductor layer doped with an element that forms a deep impurity level is used for the buried layer for the purpose of improving the operational speed of the optical semiconductor device.
FIGS. 2A-2C show an optical semiconductor structure obtained with the foregoing experiments. In the drawings, FIG. 2A shows a plan view diagram, while FIG. 2B shows a cross-sectional diagram taken along a line A-A′ shown in FIG. 2A. Further, FIG. 2C shows a cross-sectional diagram taken along a line B-B′ shown in FIG. 2A.
Referring to the cross-sectional diagrams of FIGS. 2B and 2C at first, the optical semiconductor device is formed on an InP substrate 21 of n-type, and there is formed an InGaAsP active layer 23 on a lower cladding layer 22 of n-type InP, and an upper cladding layer 24 of p-type InP is formed further on the foregoing InGaAsP active layer 23. Further, there is formed a contact layer of p-type InGaAs on the upper cladding layer 24.
Further, a mesa stripe 22M is formed so as to reach the lower cladding layer 22 by patterning the layered structure thus formed, and there are formed semi-insulating InP buried layers 26 doped with Fe at the right side and left side of the mesa stripe 22M. Thereby, as shown in FIG. 2C, the semi-insulating InP buried layer 26 also covers a beam exit end of the mesa stripe 22M.
In this experiment, growth of the semi-insulating InP buried layer 26 is achieved by an MOCVD process, wherein methyl chloride CH3Cl is added to the source material such as TMIn (trimethyl indium) or phosphine (PH3) used at the time of the MOCVD process. Further, in order to realize the desired semi-insulating nature of the InP buried layer 26 by way of pinning the Fermi level thereof, ferrocene (Cp2Fe) is added.
Thus, by adding the material of the system of organic chloride at the time of deposition of the InP buried layer, movement distance of In atoms and P atoms on the surface of the underlying layer is increased, and there is caused extraordinary increase of growth rate in the lateral direction over the primary growth surface of (311)B surface, or the like. As a result, the InP buried layer 26 grows rapidly in the right and left directions from the mesa stripe 22.
As are result, there is formed a characteristic cross-sectional structure for the InP buried layer 26 as shown in FIG. 2B including a flat part 26A defined by a (100) surface extending along the mesa stripe 22M with a certain width and sloped regions 26B and 26C rapidly decreasing the layer thickness at respective outer sides of the flat part 26A. With such a structure, it is possible to suppress the formation of eaves as shown in FIG. 1 even when the extending direction of the mesa stripe 22M is deviated from the <011> direction.
On the other hand, with the structure formed by such a process, it has been discovered that there occurs a growth that forms a principal growth surface of (411)A surface at the exit end of the mesa stripe 22M as shown in FIG. 2C. Because of the formation of such a sloped part 26E adjacent to the flat part 26D, there arises a problem in that it becomes difficult to inject the optical beam exited from the mesa stripe 22M into the optical waveguide such as an optical fiber in the event an optical window is formed in the exit direction of the mesa stripe 22M.
While formation of such a sloped part 26D can be resolved when the growth of the InP buried layer 26 is continued over a prolonged time, such an approach wastes the source material and tends to invite various problems such as degradation of production throughput, frequent equipment maintenance, and the like. Thus, such an approach is deemed not realistic to mass production of semiconductor devices.